The present invention generally relates to satellite communication systems. In particular, the present invention relates to controlling the uplink power in a satellite communication system.
Satellites have long been used to provide communication services to large regions of the globe. Historically, communication satellites have used frequencies in the range of 3 to 12 GHz (C or Ku band) to produce an antenna beam which covers a large portion of a continent. Modern satellites may operate at frequencies of 20 to 30 GHz (Ka band) to produce a beam which may cover an area (or xe2x80x9ccellxe2x80x9d) with a diameter of 300 to 400 miles. Many such cells may be needed to provide communications to a region which previously may have been serviced by a single antenna beam. A modern cellular communication satellite may employ many antennas to generate a large number of beams used for transmitting downlink signals to, and receiving uplink signals from, various User Earth Terminals (UET) distributed over the surface of the earth.
In order for communication to occur on the uplink, signals generated by the UET must be of sufficient power to be received by the satellite. Thus, the antenna gain of the satellite""s uplink antenna coupled with the transmission power of the UETs must be sufficient to allow communication to occur. Typically, communication satellite systems are designed with a predetermined, fixed satellite uplink antenna gain. Thus, the transmission power of the UET is typically controlled to enable and ensure communication.
In practice, several factors exists which may negatively impact the uplink communication channel. That is, certain undesired influences may cause the actual antenna gain to vary from the predetermined, designed antenna gain or may cause attenuation of, or interference with, a signal transmitted by a UET. For example, antenna gain may be affected by gain roll-off which may cause the antenna gain to vary spatially over the cell or, alternatively, antenna gain may vary over the cell as a result of pointing errors in the antenna. Atmospheric attenuation, also known as xe2x80x9crain loss,xe2x80x9d or interference among several UETs, also known as Co-Channel Interference (CCI), may also affect the quality of a signal transmitted from a UET. Each of these conditions, gain roll-off, antenna pointing errors, atmospheric attenuation, and CCI is further discussed below.
I . Gain Roll-Off
The pattern of cells on the surface of the earth is known as the cellular pattern of the satellite communication system. The cellular pattern in a modern satellite communication system may be defined on the surface of the earth such that the maximum gain of a satellite antenna beam is directed toward the center of its assigned cell. The boresight of a satellite antenna beam may be defined as the maximum gain point in the satellite antenna beam, and is typically directed to the center of a cell. The edge of a cell may be defined by determining the angular deviation from the antenna boresight at which the gain of the antenna beam- drops to a predetermined value below the maximum gain value, typically at least 3 dB below the maximum gain value. The decrease in antenna beam gain with increasing angular deviation from boresight is known as gain roll-off. In terms of uplink power, a communications signal which is transmitted to the satellite from a UET located at the edge of a cell may be received by the satellite antenna with a gain which is at least 3 dB lower than the gain of a signal which is transmitted from a UET located at the antenna boresight, or center of the cell. Thus, the transmission power level of a terminal located at the edge of a cell must be at least 3 dB higher than that of a terminal located at the center of a cell in order to achieve the same level of performance. In other words, if the edge of a cell is defined as the angle from boresight at which the satellite antenna gain has decayed to 3 dB below the maximum antenna gain at the boresight, a UET at the edge of the cell may need to use a transmission power level 3 dB higher than a UET at the center of the cell in order to compensate for the reduced antenna gain at the edge of the cell. By transmitting at the 3 dB higher transmission power level, the signal from the UET at the edge of the cell may be received at the satellite with a power that is approximately equal to the power of a signal from the UET at the center of the cell. In order to simplify and reduce the cost of uplink components installed on the satellite, it is desirable to maintain a similar received power level for each UET in the cell. Thus, it is desirable to modify the transmission power of each UET in the cell to compensate for any reduction in the antenna gain at each UET resulting from the UET""s position within the cell.
2. Antenna Pointing Errors
In practice, the antenna beams of a cellular communication satellite are generally not directed precisely toward the centers of their assigned cells. Slight mis-orientations of the antenna boresights and deviations from a perfectly circular, zero-inclination satellite orbit give rise to pointing errors. These pointing errors may cause the location of the maximum gain of an antenna beam to deviate from the cell center. Some pointing errors may also cause the maximum gain of an antenna-beam pattern to change measurably over the course of a day. In other words, the antenna beam gain distribution across the cell may change with time.
The antenna beam gain at the edge of a cell typically rolls off rapidly as the distance from the center of the cell increases, that is, as the angular deviation from boresight increases. Thus, a pointing error corresponding to only 10% of a cell diameter may cause the antenna beam gain at the edge of a cell to vary by 2 dB or more. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for antenna beam pointing errors.
3. Atmospheric Attenuation
Achieving satisfactory communication performance for a signal transmitted from a UET to a satellite generally depends upon receiving a requisite level of signal power at the satellite. That is, each user terminal must transmit a signal with sufficient power to be received. The relationship between the power of the signal transmitted by the terminal and the power of the signal received by the satellite receiver depends in part upon the amount of attenuation of the signal as it passes through the earth""s atmosphere. At Ka-band frequencies, the amount of atmospheric attenuation varies considerably as meteorological parameters and weather patterns change. In particular, the occurrence of rain has a pronounced effect on the attenuation of a Ka-band communication signal. The attenuation of the communication signal is known as rain loss or rain fade, although other meteorological phenomena may also provide attenuation. Such atmospheric conditions and/or weather patterns may change rapidly and may vary among different UETs in a cell depending upon the UET""s position within the cell. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for the attenuation experienced by the UET""s signal due to rain loss.
4. Co-Channel Interference
Immediately adjacent cells in a cellular satellite communication system typically use different frequencies for transmitting signals. However, non-adjacent cells may use the same frequency. Such frequency re-use among cells within a cellular pattern serves to reduce the overall frequency bandwidth necessary for the satellite communication system. However, imperfections in satellite antenna beams such as, for example, sidelobe generation, may cause signals transmitted from a UET located in a first cell to be received by a satellite antenna beam which is assigned to receive signals from UETs located in a second cell which uses the same frequency as the first cell. Signals transmitted by UETs located in different cells but using the same frequency may thus interfere with each other, and may cause degraded communication performance. That is, a desired signal received by the satellite from a first UET may be interfered-with by signals from other UETs in other cells using the same frequency as the first UET. The interference from the other UETs may interfere with the desired signal and may adversely affect the performance of the communication system. The interference from other UETs is often referred to at Co-Channel Interference (CCI).
The ratio of the signal power received from the desired UET to the background noise is known as the signal-to-background ratio (SBR). The number of errors in a data signal received from a UET at a satellite (i.e., the error count) may be impacted by the SBR. The background may include thermal and other noise sources as well as interference sources such as interference from other UETs using the same frequency. In order for the satellite to receive a signal from a particular UET, the transmission power of the UET must be sufficient to provide at least a certain desired minimum SBR. As the background portion of the SBR increases with increasing CCI, the signal portion of the SBR is also increased to maintain the desired SBR. That is, the UET of interest transmits with increased transmission power to maintain the desired SBR in light of the increasing interference from other UETs. However, increasing the transmission power of the UET of interest raises the background level for the other UETs. The other UETs, also seeking to maintain the desired SBR, in turn respond by raising their transmission powers. The UET of interest may react by further increasing its power, and so on until all terminals in the system are operating at the maximum transmission power. This phenomenon is known as system runaway.
Satellite systems have been proposed that attempt to address the problem of system runaway by establishing a single, constant transmission power level for each UET. These proposed systems contemplated using frequencies in the range of 3 to 12 GHz (C or Ku band). Maintaining a constant power for each UET may be acceptable at Ku or C band frequencies in some cases. However, at higher, Ka-band frequencies (20-30 GHz), for example, attenuation alone may cause the power of the received signals at the satellite to vary over a range of 20 dB or more. A comparable dynamic range would be required of the satellite demodulator, which would have a dramatic impact on system complexity and cost. Additionally, such a system would produce a high degree of CCI and increased power consumption. Because of the high CCI, the maximum tolerable interference level from other UETs would unduly limit the number of UETs that may be used, and system capacity would be needlessly limited. Therefore, it is desirable to maintain satisfactory communication performance (typically, maintain a desired SBR and/or a desired error count) while preventing system runaway.
Additional complexity arises in an uplink power control system with regard to UETs which transmit data intermittently rather than continuously, or whenever a UET first establishes a communication channel for transmission to the satellite. When a UET initiates a transmission, the UET may be forced to send an uplink signal into an attenuation and interference environment substantially unknown to the UET. That is, the UET may not be able to transmit initially with a transmission power that provides the desired SBR while not providing needless CCI to other UETs using the same frequency. If the initial transmission power is set too low, the signal may not be received by the satellite. If the initial transmission power is set too high, it may add a disproportionate amount of CCI and degrade the quality (adversely impact the SBR) of other uplink signals in the system.
A pending application entitled xe2x80x9cComprehensive System and Method for Uplink Power Control in a Satellite Communication Systemxe2x80x9d, application Ser. No. 09/596,683, filed Jun. 19, 2000, presented a method and means for controlling uplink transmission power in a satellite communication system to compensate for gain roll-off, antenna pointing errors, atmospheric attenuation, and co-channel interference. In operation, as shown in FIGS. 1 and 7, the uplink transmission power level used by a UET 110 for transmitting a traffic burst is continuously adjusted based on the number of errors detected and corrected for a prior traffic burst by the error detector/decoder 711 of a processing satellite 140. The success of the method depends upon the ability to determine a needed correction to the uplink transmission power for a traffic burst from a signal fidelity measurement made on a prior traffic burst. The method is therefore optimum only for systems utilizing a signaling scheme in which each UET transmits a large number of traffic bursts on a given assigned frequency channel and time slot. In this case, interference between UETs would vary slowly, and the transmission power level for a traffic burst may be reliably determined from a signal fidelity measurement made on a prior traffic burst. Some signaling schemes, however, may be characterized by large and rapid variations in interference between UETs. In the demand assigned multiple access (DAMA) signaling scheme, for example, a UET may be assigned a different frequency channel and time slot for each traffic burst to be transmitted, so that the level of interference experienced by any given traffic burst bears no relation to that experienced by any prior burst.
Since the frequency channel and time slot (xe2x80x9cchanslotxe2x80x9d) assigned to a terminal for the transmission of traffic bursts may change from burst to burst in the demand assigned multiple access (DAMA) signaling scheme, it is not generally possible to determine the proper transmit power level for a traffic burst based on measurements of signal fidelity made on prior traffic bursts. It is therefore critical to overall system performance that the power leveling thresholds and traffic burst power offsets assume values which result in a high probability that the traffic bursts will be received by the satellite at or above a desired level of signal fidelity. At system turn-on, the thresholds and offsets may be assigned values based on theoretical analysis of expected system performance. In general, however, the optimum values of thresholds and offsets may not be known until the system is operational and the dependence of system performance upon these values may be observed. The values may be different for each beam due to differences in receiver electronics, and may change over the life of the system due to component degradation.
A technique for calibrating the values of thresholds and offsets used in uplink power control is desired which is based on observed system performance, which allows optimum values to be determined for each beam independently, and which automatically updates the values over the life of the system. This invention addresses these problems and provides a solution.
The preferred embodiment is useful in a satellite communication system comprising a satellite arranged to receive data carried by an uplink signal having a received power and to make a comparison of the received power of at least a portion of the uplink signal with a power threshold. The system also comprises a UET equipped with a transmitter arranged to transmit the uplink signal at a transmit power adjusted at least in part in response to the comparison. In such an environment, the power threshold preferably is controlled by determining errors in the data and adjusting the power threshold in response to the determining of the errors. The determining of errors and adjusting preferably are carried out by one or more processors. By using the foregoing techniques, the power threshold can be effectively controlled in the presence of DAMA signaling.
By using the foregoing techniques, the system may provide self-calibration for maintaining a desired level of signal fidelity. Threshold values may be automatically adjusted based on direct observations of system performance. The system may compensate for degradation of receiver electronics components, and may react to changes in the average interference environment.